In a case where a thermoplastic resin composition is used for various applications such as housings of a personal computer and a display device, an electronic device material, automotive exterior and interior parts, and the like, there sometimes occurs a problem that it is difficult to release generated heat because a thermoplastic resin composition is less thermally conductive than an inorganic substance such as a metal material. In order to solve the problem, an attempt has been extensively made to obtain a highly thermally conductive resin composition by blending, with a thermoplastic resin, a highly thermally conductive inorganic compound in a large amount. In order to obtain a highly thermally conductive resin composition, it is necessary to blend, with a resin, a highly thermally conductive inorganic compound such as graphite, carbon fiber, alumina, or boron nitride generally in an amount of not less than 30 vol % and further in an amount of as many as not less than 50 vol %. However, a resin, with which a highly thermally conductive inorganic compound is blended in a large amount, cannot be molded by use of a general-purpose mold. Therefore, in order to reduce an amount of highly thermally conductive inorganic compound to be blended with a resin, the resin itself is required to be more thermally conductive.
From such a viewpoint, an object to achieve a highly thermally conductive organic material is extremely important. Japanese Patent Application Publication, Tokukaisho, No. 61-296068 A (1986) discloses, as a method for achieving a highly thermally conductive organic material, a plastic compound which is filled with extremely highly oriented polymer fibers and is highly thermally conductive. This employs a characteristic such that extremely highly oriented polymer fibers disclosed in POLYMER, Vol. 19, P 155 (1978) have a higher thermal conductivity in their fiber axis direction.
However, extremely highly oriented polymer fibers have a lower thermal conductivity in a direction perpendicular to their fiber axis direction. Therefore, even random dispersion of such extremely highly oriented polymer fibers into an organic insulating composition causes less improvement in their thermal conductivity.
As described earlier, an orientation of polymer fibers in an organic insulating composition in one direction makes it possible to obtain an organic insulating material which is highly thermally conductive in the direction in which the polymer fibers are oriented. However, on the contrary, there is a problem that the organic insulating material is less thermally conductive in a direction other than the direction in which the polymer fibers are oriented.
ADVANCED MATERIALS, Vol. 5, P107 (1993) and Specification of German Patent Application Publication No. 4226994 describe that a monomer such as diacrylate which contains a mesogenic group is oriented in a given direction and then subjected to a cross-linking reaction, so as to obtain an anisotropic material which has a higher thermal conductivity in an in-plane direction of a film in which molecular chains are oriented. However, the anisotropic material has a low thermal conductivity in a direction other than the in-plane direction, especially in a thickness direction of the film.
In general, there are overwhelmingly many cases where a film material is employed so that heat is transferred in a thickness direction thereof. Therefore, a material whose thermal conductivity is low in a film thickness direction is less effective in thermal conduction.
On the other hand, a method for orienting molecular chains in a thickness direction also has been studied. Japanese Patent Application Publication, Tokukaihei, No. 1-149303 A (1989), Japanese Patent Application Publication, Tokukaihei, No. 2-5307 A (1990), Japanese Patent Application Publication, Tokukaihei, No. 2-28352 A (1990), and Japanese Patent Application Publication, Tokukaihei, No. 2-127438 A (1990) describe methods for producing an organic material such as polyoxymethylene or polyimide while applying an electrostatic pressure.
Japanese Patent Application Publication, Tokukaisho, No. 63-264828 A (1988) describes a material obtained as follows: Sheets in each of which molecular chains of polypropylene, polyethylene, or the like are arranged are adhered to be laminated with each other so that orientation directions of the molecular chains are aligned with each other. Then, the laminated sheets are thinly sliced in a perpendicular direction to the direction in which the molecular chains are arranged, so as to obtain the material in which the molecular chains are arranged in the perpendicular direction. It is true that such a method makes it possible to obtain a material which has a higher thermal conductivity in a film thickness direction. However, molding of such a material is extremely complicated and thus such a material is limited in use.
Epoxy resins described in Japanese Patent Application Publication, Tokukai, No. 2003-268070 A and Pamphlet of International Patent Application Publication No. 2002/094905 or bismaleimide resins described in Japanese Patent Application Publication, Tokukai, No. 2007-224060 A and Pamphlet of International Patent Application Publication No. 2006/120993 are thermally conductive to some extent but have a disadvantage in that they have complicated molecular structures difficult to produce.
A thermoplastic resin is exemplified by a resin molded article described in Japanese Patent Application Publication, Tokukai, No. 2008-150525 A. The resin molded article is obtained as follows: Thermal liquid crystal polyester is oriented by at least one external field selected from a flow field, a shear field, a magnetic field, and an electric field, so as to cause the thermal liquid crystal polyester to be highly thermally conductive in a direction in which the thermal liquid crystal polyester is oriented. The resin molded article is highly thermally conductive in one axis direction but less thermally conductive in the other two axis directions. In addition, in the case of the magnetic field, not less than 3 teslas of magnetic flux density is required to obtain a desired thermal conductivity. This makes it difficult to produce the resin molded article.
There have been no other examples of research report on a thermoplastic resin which is not subjected to a special molding process such as extension or magnetic field orientation and in which a resin per se is highly thermally conductive. As for a liquid crystalline thermoplastic resin, Non Patent Literatures 3 through 7 describe alternating polycondensation products of mesogenic groups and alkyl chains which alternating polycondensation products show liquid crystal phases. However, as for thermal conductivities of such polymers, none of the Non Patent Literatures describe the technique of obtaining a resin molded composition by blending, with a resin, another blended substance such as an inorganic filler.